A star that has a retinue of nine worlds has been revealed by an astronomer analyzing data from the HARPS spectrograph, which is attached to the European Southern Observatory's 3.6-meter telescope at La Silla, Chile. But the planetary system is not a new find. Neither were seven of the planets.
Astronomer Mikko Tuomi of the University of Hertfordshire found two exoplanets in addition to confirming the seven that were discovered circling the nearby yellow-dwarf star HD10180 in 2010. He did so by reanalyzing the data where the other planets were found, using the radial velocity detection method.
The method employees studying the radial velocity of a star with respect to the Earth by analyzing its Doppler shift (moving toward or away from the Earth). The star's own orbital path is altered by the gravitational pull of its attending planets.
"In addition to these seven signals, we report two additional periodic signals that are, according to our model probabilities ... statistically significant and unlikely to be caused by noise or data sampling or poor phasecoverage of the observations," Tuomi wrote as part of the research paper (PDF) accepted for publication in the journal Astronomy & Astrophysics.
HD10180 already had the distinction of being the star with the largest planetary system outside our own. Scientists studying the data found that the star, which is very much like the Sun, had five and possibly seven planets in its train. Five of the worlds were estimated to be in the mass range of Neptune. One of the extra-solar planets was found to orbit at a great distance from the star, was the size of Saturn, and had a mass estimated between 12 and 25-times that of the Earth. The seventh planetary object was estimated to be a "super-Earth" (with a mass of 1.4 that of Earth) but still considerably smaller than its sister worlds.
The two planets Tuomi found are also smaller worlds than most in the system. They are also "super-Earths." One is 1.9 times more massive than Earth and the other is 5.1 Earth-masses. Along with the other "super-Earth," both are also in tight orbit around the parent star, too close to be optimal for sustaining life as we know it (in the "habitable zone"). The smaller world whirls around HD10180 in 2.1 days. The two newer discoveries orbit their host in 10 and 68 days.
Technically speaking, however, with its nine planets the HD10180 system isn't a twin of the Solar System with nine planets. With the downgrade of Pluto to a dwarf planet, the Solar System only has eight actual planets and three dwarf planets. It is also home to three planets smaller than Earth. As noted all of HD10180's planets are larger than Earth.
Thus far, given the incredibly vast distances be surveyed, finding worlds the size of Earth has been difficult. Only one world smaller than Earth has been detected.
The first two worlds ever discovered to be close to Earth's size were announced online in Nature in December. They also orbited a Sol-like star, Kepler-20. Both planets, with masses of 1.03 and 0.87, circled their parent in tight orbits inside what is considered the habitable zone.
(photo credit: ESO/L. Calçada, Creative Commons)